1. Field of the Invention
The present invention relates to subsurface well bore equipment and more particularly to an apparatus for milling away tubular conduits such as liners encased within well bores.
There is a special need in the oil and gas industry for tools which can remove the casing in an oil and gas well, drill collars, drill pipe and jammed tools. This is accomplished from the surface with a tool on the end of a drill string. The drill string can range from hundreds to thousands of feet in length. Typically, the working area of the milling tool in a well is from three to ten thousand feet or more below the surface. In various operations at this subsurface point, a portion of the well casing may have to be removed so that drilling can be conducted in a different direction or a drill collar may have to be removed. One reason to remove casing is to permit the drilling of an additional well from the main well. Another use for the milling tools is to remove a tool jammed in the well. This latter use entails destroying the tool by milling through the tool and the borehole. This, then, reopens the hole so that drilling may be commenced.
2. DESCRIPTION OF THE PRIOR ART
Milling tools have been used for many years in subsurface operations. Many of these tools have a lower pilot or guide section and an upper cutting section. These tools include pilot mills, drill pipe mills, drill collar mills and junk mills. These mills all have one thing in common, and that is, to remove some material or item from a well hole. Each of these mills accomplishes this function in the same way by reducing the item to shavings, hence, small chips.
The various mills in use have different types of cutter blades. Most of these cutter blades, however, are permanently fixed to the outside surface of the tool by, for example, welding blades on the outer casing to perform the milling function. Once these blades are worn through, the milling tool then has to be replaced. This includes the entire body and connectors associated with the mill.
The prior art is replete with examples of milling tools. An early example of a milling apparatus is found in U.S. Pat. No. 2,855,994. This patent illustrates a number of radially extending milling blades that are metallurgically bonded to the outer casing of the body of the milling apparatus. The blades of the milling tool are oriented with respect to the length of the milling tool at different elevations such that the tool continues to perform the cutting function without flaring the pipe that the tool is cutting as the blades wear out.
As heretofore indicated, once these blades wear out the tool needs to be replaced with a new tool.
Another more recent patent relating to milling tools is U.S. Pat. No. 4,717,290. This milling tool consists of a tool body which has a plurality of cutter blades extending from the body. Each cutter blade has a negative axial rake and essentially constant negative radial rake. Each cutter arm has a close packing of cylindrical cutting grade tungsten carbide inserts, each of the inserts being set at a lead angle of from 0 to 10 degrees. Each of the blades radially extending from the body of the milling tool is oriented in a spiral, or angled pattern, one from the other; each of the blades being equidistantly spaced around the body of the tool.
Again, as these blades wear away, the entire milling tool needs to be replaced including the body and the connecting ends, etc.
There is a whole family of milling tools that have movably expandable arms that extend radially out from the body of the milling tool, the extending operation occurring downhole. U.S. Pat. No. 3,105,562 is typical of these expanding type reamers and milling tools.
The present invention obviates the need to replace the entire body of the milling apparatus by providing replaceable blades for the milling tool. The milling tool of the present invention is comprised of several components that when assembled, firmly locks a series of milling blades through slots in the body of the milling tool. When the blades become worn, the tool is simply disassembled, new blades are inserted through slots formed by a cylindrical housing from the inside of the housing and a central mandrel is then inserted within the housing, thereby locking each of the replaceable blades in place for further milling operations.
The present invention, therefore, has an advantage over the prior art in that the cutting blades are easily replaceable.
Still another advantage of the present invention over the prior art is that different types of milling blades may be utilized in the same body of the apparatus.
Yet another advantage of the present invention over the prior art is that the blades are mechanically locked in place thereby obviating the need to weld the blades to the housing thereby compromising the integrity of the base metal of the blades and the cutting material secured thereto.